The invention relates to a method and an apparatus for controlling the operational behavior of an internal combustion engine in a predetermined domain of operation. The control process includes measuring the dispersion of cyclic fluctuations of the mean combustion chamber pressure in time intervals which are synchronous with the engine r.p.m. Based on the results of this measurement, the fuel-air mixture and/or the quantity of recycled exhaust gas is changed. An electrical signal corresponding to the non-uniform operation of the engine and an artificial signal identifying the uniform operation of the engine are compared and the phase relation of these two signals is used as a measure of the controlled variable.
At the present time, great effort is expended to permit the operation of internal combustion engines in a domain in which the exhaust gases are free of noxious components and/or fuel consumption is as small as possible so as to comply with increasingly stringent regulations regarding exhaust gas concentrations and to take account of the overall fuel supply situation.
One possibility is to operate the engine with as lean a fuel-air mixture as possible, i.e., to adjust the engine in the direction of a lean mixture, because, in that region, one may respect a relatively innocuous exhaust gas and low fuel consumption although the concentration of nitrogen oxide increases. For this reason, it is of great significance to know as precisely as possible what the lean running limit of the engine is. This determination may be made, for example, on the basis of the fluctuations of the pressure in the cylinders of the engine. It is known that an internal combustion engine runs less smoothly the farther away its operation is from a stoichiometric fuel-air ratio (air number .lambda. = 1).
When this phenomenon is subjected to a detailed analysis, it is seen that the individual pressure fluctuations depend on uncontrollable operational parameters of the engine, namely on fluctuations in the air number, charge and air turbulences and similar influences. When the combustion chamber pressure is measured as an instantaneous value as a function of the angular speed of the crankshaft, there are further disturbing effects, for example due to oscillating masses of the crankshaft, uneveness of the roadway or other forces which have an effect on the engine block of the internal combustion engine. Thus, it is impossible to use the phenomenon of smooth running of an engine to obtain a suitable control variable because the superimposed fluctuations and disturbances are a hindrance. These disturbances could, in principle, be filtered out with low-pass or other filters, but even the use of filters is problematical because the engine is to be used in a wider r.p.m. domain. It is difficult to find suitable filters for both high and low frequencies.